We have developed a quasi-adiabatic calorimeter for measuring directly the electrocaloric effect (ECE) from 4.2 to 380 K. The setup primarily consists of an outer vacuum chamber (OVC) and an inner chamber. The OVC is sealed by indium junctions and its pressure stabilizes to around 10-6 mbar, or below, during the measurements. The inner chamber is delimited by a thermal shield that provides the temperature control via a resistive heater and a thermocouple. The thermal shield is mechanically anchored to an oxygen-free copper block, whose temperature is continously monitored during experiments. Inside the inner chamber, the disc-shaped sample is in “contactless” mode, that is, the sample is held in place by the electrical wiring only.
Top photograph: Inside of the inner chamber.
On the disc-shaped sample, the voltage is applied to one side with a high-voltage (HV) coaxial line, while a grounded wire is contacted to the opposite side. We use silver epoxy for the electrical contacts and a homemade voltage source, whose output is 0 – 5 kV. A K‑type thermocouple, made of extremely thin leads to minimize thermal losses and bonded to the grounded side of the sample by a thermally conductive electrically insulating adhesive, detects the temperature difference between sample and copper block. All electronics are controlled with a computer, so that the whole measuring process is operated automatically.
Top figures: Representative direct electrocaloric measurements for Ba0.85Ca0.15Ti0.94Hf0.06O3, for several applied voltages, V. Left: Raw data as a function of time, for T = 300 K. Right: Adiabatic temperature change from direct (filled symbols) and indirect (empty symbols) measurements. The indirect data are calculated from polarization-electric field hysteresis loops, using standard procedures that -frankly- do not inspire much confidence.
For any set temperature, the main measurement protocol consists in applying voltage pulses, while monitoring the temperature difference, ΔT, between sample and copper block (i.e. thermal bath) at any time t. Following the on/off switching of the electric field (i.e. E = V/d, where d is the sample thickness), the ECE kicks in almost instantly, therefore quasi-adiabatically. After each jump, the temperature of the sample relaxes towards the temperature of the thermal bath. The temperature change for truly adiabatic conditions, ΔTad, is obtained from fitting the ΔT (t ) relaxation. The mass of the addenda (silver epoxy, alumina adhesive, wires) is typically less than 5% of the sample mass. Accordingly, the addenda heat capacity is negligible with respect to the sample contribution and corrections to ΔTad can safely be disregarded.
David Gracia, Sara Lafuerza, Javier Blasco, and Marco Evangelisti.